1. Field of the Invention
The present invention is directed to efficient acoustic (sonic and ultrasonic) bulk wave transducers for operation in at least two frequency bands. Applications of the transducers are for example, but not limited to, medical ultrasound imaging, nondestructive testing, industrial and biological inspections, geological applications, and SONAR applications.
2. Description of the Related Art
The utilization of the nonlinear elasticity of tissue and ultrasound contrast agent micro-bubbles in medical ultrasound imaging provides improved images with less noise. The widest use is in the so-called harmonic imaging, where the 2nd harmonic component of the transmitted frequency band is used for the imaging, extracted from the signal either through filtering or through the Pulse Inversion (PI) technique. A use of 3rd and 4th harmonic components of the transmitted pulse for imaging is also presented in U.S. Pat. No. 6,461,303.
U.S. Patent Application Publication No. 2005/0277835 and U.S. Patent Application Publication No. US 2006/0052699 A1 describe in depth different uses of dual band transmitted ultrasound and acoustic pulse complexes that provide images with reduced noise, images of nonlinear scattering, and quantitative object parameters that greatly enhance the use of ultrasound and acoustic imaging. The methods are applicable both with transmission and scatter imaging. For these applications, dual band pulse complexes are used, such as, for example, illustrated in FIGS. 1a and 1b. In FIG. 1a a high frequency (HF) pulse 101 rides on the peak pressure of a low frequency (LF) pulse 102. FIG. 1b shows another situation where the HF pulse 103 rides on the maximal gradient of the LF pulse 102. The ratios of the center frequencies of the LF and HF pulses are typically in the range of 1:5-1:20, and at the same time the HF pulse must be found in defined intervals of the LF pulse throughout defined depth ranges of the images.
In other applications, the same probe is required to transmit a low frequency wave (e.g., 0.5-2 MHz) for treatment of tissue (hyperthermia or cavitation destruction of tissue) or release of drug carried in nano or micro particles or bubbles, while being able to provide ultrasound imaging from the same probe surface at a higher frequency (e.g. 5-10 MHz). In yet another application, a probe is required for combined ultrasound treatment and imaging with 3 frequency bands, where a 2nd lower frequency (LF2) band ˜400 kHz is used, for example, to generate pulses for cavitation in the tissue to break nano-sized liposome particles containing drugs for drug delivery to tumors, a 1st low frequency (LF1) band ˜3 MHz is used for heating of the tissue for hyperthermia treatment of tumors (often referred to as HIFU—High Intensity Focused Ultrasound) or to increase blood flow in the tumor for improved oxygenation of the tumor or to improve the efficiency of the ˜400 kHz breaking of drug carrying particle, and a high frequency (HF) band ˜20 MHz is used for imaging, potentially also in combination with the ˜3 MHz LF1 band for nonlinear manipulation of object elasticity for imaging, for example according to U.S. Patent Application Publication No. 2005/0277835 and U.S. Patent Application Publication No. US 2006/0052699 A1.
In yet other applications, a larger selection of frequency bands is required to be available for imaging from the same probe for a large variation of depth ranges. For example in portable ultrasound imaging systems for emergency medicine, center frequencies of 2.5 MHz are used for deep object imaging, and the same probe should be able to image at 7-10 MHz center frequencies for objects closer to the body surface. The arrays can for example be arranged as phased linear arrays, switched linear arrays, and curvilinear arrays. The need for multiband transducers is also found in many other applications of acoustic imaging such as, for example, in non-destructive testing (NDT) of materials, observations of geological formations with elastic waves, and SONAR measurements and imaging of fish, for example close to the sea bottom, the sea bottom, and objects like mines both on the sea bottom and buried under the sea bottom or in the soil on land. This both relates to nonlinear measurements and imaging with multiband pulse complexes, and the ability to select different frequency band pulses for different needs, such as different measurement ranges.
Dual band transmitted pulses were used in M-mode and Doppler measurements in Br Heart J. 1984 January; 51(1):61-9. Further examples are shown in U.S. Pat. No. 5,410,516 where sum and difference bands of the transmitted bands produced in the nonlinear scattering from contrast agent micro-bubbles where detected. A further development of this dual band transmission is done in U.S. Pat. No. 6,312,383 and U.S. patent application Ser. No. 10/864,992.